Ukemi is quite difficult to practice. Practicing the role of uke allows us the time to study openings in our own attacks, and teaches us ways of safely defending those openings. It is the responsibility of uke to recognize openings for atemi and to respond to those openings. Sensitive ukemi, wherein we follow the movement of our training partner, develops the sensitivity required to change directions appropriately and when indicated, which makes henka waza (transitioning techniques) and kaeshi waza (reversing techniques) possible during later stages of practice. Sensitive ukemi is the means by which we may "steal" the techniques of our teachers and senior students.
Most of the time, sempai will slow way down when training with new, or inexperienced training partners. The techniques are action-response katas, and both roles have delineated actions. Fast practice, or practice that transitions from one technique to another (henka waza), etc. requires, for safety, that both practitioners be fluent in both roles of a wide range of techniques. Slow movement can amplify mistakes, however, which may be tempting to attempt to resist.
Many beginners (and even some experienced practitioners) have a tendency to resist a technique by either tensing up against it, or disconnecting and changing the direction of their ukemi. There are techniques (henka waza) for dealing with such systems of resistance, but those are reserved for explicit demonstration and practice. Most instructors would like the students to practice just what has been demonstrated, at a level of intensity that is appropriate for the less experienced of each partnership.
Many instructors expect to be the only instructor on the mat. If your sempai aren't explaining things, it could be that such behavior is not tolerated in your dojo. It's not necessarily the case everywhere, but it is definitely the case some places.
You really should keep practicing. The most practical aspect of aikido really is ukemi. With experience, you will develop the sensitivity required to sense changes in direction by an obstinate attacker, and adjust your application of technique appropriately. The far more likely scenario to being attacked, though (unless you are in some kind of high-risk occupation or social situation), is that you will, at some point, fall down. Falls become tremendously more dangerous as we age, but, with practice, we can improve how we fall.